index.lock: File exists.

$ g add app
fatal: Unable to create '~/project_dir/.git/index.lock': File exists.

Another git process seems to be running in this repository, e.g.
an editor opened by 'git commit'. Please make sure all processes
are terminated then try again. If it still fails, a git process
may have crashed in this repository earlier:
remove the file manually to continue.

なにこれ？？？

ja.exploratory.io

qiita.com

削除してもだめという説明もあるけど、手元は削除したらうまくいった。